Due to increasingly strict governmental emission regulations, efforts to reduce emissions from IC engines have increased. These regulations have posed, and will continue to pose, particular problems for diesel engines, which while being very efficient, often produce very high emissions of NOx and particulate matter (“PM”). After treatment of diesel engine exhaust is often needed to help reduce diesel engine emissions, because diesel engine emissions cannot be sufficiently reduced by modification of the engine design alone. This after treatment typically includes the use of a diesel particulate filter (“DPF”) and/or a NOx trap.
The DPF is typically a ceramic filter placed in the exhaust stream. The PM, which primarily consists of carbon particulates, collects in, and on, the DPF and clogs the filter causing increased backpressure that reduces engine efficiency or makes the system inoperable. Thus, the DPF must be regenerated to maintain its effectiveness.
A NOx trap is typically placed in the exhaust stream and has an absorbent that absorbs the NOx from the exhaust stream. When the absorbent has reached its maximum absorbing capacity, no more NOx will be absorbed. Thus, like the DPF, the NOx trap must be regenerated to maintain its effectiveness.
DPF regeneration usually occurs when the carbon particulates collected in, and on, the DPF are combusted when the DPF temperature rises past a minimum threshold. This increase in DPF temperature is typically a direct result of an increase in exhaust temperature when the engine is operated at high load. However, when engine operation does not provide engine exhaust temperatures high enough to regenerate the DPF, the exhaust stream or the DPF must be heated by other methods. For example, fuel may be injected into the exhaust stream and combusted on the DPF or on a fuel combustor placed upstream of the DPF to heat the exhaust stream. However, the use of fuel for this purpose results in an increased fuel penalty.
A NOx trap typically contains an absorbent-catalyst system that provides the multiple functions of NOx trapping, NOx release, and NOx reduction. Total NOx emissions are decreased by operating the NOx trap in one or more cycles, each consisting of trapping and regeneration cycles. Usually one component of the NOx trap absorbs NOx from the exhaust stream during the trapping cycle under oxidizing conditions, or conditions where the exhaust stream contains excess oxygen. This component is typically selected so that when the exhaust stream is made reducing, that is, the exhaust stream composition is adjusted to contain excess reductant (e.g., fuel hydrocarbons, carbon monoxide, hydrogen, etc.) relative to oxygen, the NOx is released. The NOx trap also contains a NOx reduction catalyst upon which NOx reacts with a reducing agent under reducing conditions to form non-polluting N2. Thus, during the regeneration cycle, when the exhaust stream is made reducing, NOx is released and reacts with the reducing agent on the reduction catalyst to form N2.
As noted above, the NOx trap environment must be made reducing in order to convert the trapped NOx to N2. Similar to the DPF regeneration mentioned above, fuel may be injected into the exhaust stream and combusted on the NOx trap or on an upstream fuel processor in order to raise the temperature of the NOx trap, if required, consume the oxygen, and to produce the reducing environment. See for example Applicant's copending U.S. patent application Ser. Nos., 10/431,171 and 10/309,936, each of which is hereby incorporated by reference in their entirety. As described in these cases, using a fuel processor placed upstream of the NOx trap works well and can achieve reasonable fuel penalties when the fuel processor is designed to have a low heat capacity, or when the NOx trapping time between regenerations is made long. However, fuel penalty may increase when fuel processors having high heat capacities are used, or when the NOx trapping time between regenerations is short. In addition, these designs may only be effective for a limited range of exhaust temperatures.
Accordingly, it would be desirable to provide improved systems and methods for regenerating a NOx trap. Similarly, it would be desirable to provide systems and methods that could reduce the fuel penalty associated with emissions control and NOx trap regeneration. It would further be desirable to provide systems and methods that could provide such a reduced fuel penalty over a wide range of exhaust temperature conditions.